By Peter Evans 

LONDON--Black Friday used to be just another rainy day in November for retailers in the U.K., where Thanksgiving isn't a holiday. But now, the sales extravaganza is fast becoming one of the most important dates on their calendar.

Last year, many U.K. stores embraced Black Friday for the first time, albeit tentatively. This year, they have thrown off the shackles and are planning big discounts on everything from tablets to power drills to quad bikes.

Many retailers expect their biggest day of sales so far this year. And they know 2014 will be the most competitive year ever as, for the first time, giants such as Tesco PLC and Marks & Spencer Group PLC commit fully to the deals bonanza.

"Black Friday now marks the real start of Christmas shopping," said Rob Hattrell, general merchandise director at Tesco, the U.K.'s biggest retailer. The supermarket chain--which is running ads urging "bargain hunters" of the world to "unite"--plans to make reductions worth GBP15 million ($23.5 million) over three days, starting at midnight on Thursday.

The U.K. is also bracing itself another traditional part of Black Friday: stampeding shoppers racing for the best deals the moment the stores open.

After a small number of violent incidents last year, retailers now say they are prepared for the hysteria. Wal-Mart Stores Inc.-owned Asda has called in thousands of staff to work overtime on Friday and has beefed up its security detail.

Asda is hoping to make Wal-Mart's decades of Black Friday experience a competitive advantage in the U.K. Around 80% of the discounted products it offers--mainly electronic items like televisions and tablets--will be sourced from suppliers used by Wal-Mart in the U.S.

Asda will also ramp up in-store Wal-Mart branding--the only time in the year that the name of the parent company is used in the U.K.--in an attempt to generate a sense of "American excitement," said Andrew Moore, Asda's chief merchandising officer.

"We've learnt a lot from the U.S.," Mr. Moore said.

The discount frenzy will take place in stores and online. Online transactions will increase 22% from last year, according to research by Visa Europe. In total, British shoppers will spend more than GBP1 million every three minutes.

Amazon.com Inc. is running seven days of Black Friday deals, having bombarded customers with reminders for weeks in advance. The company, which claims it introduced Black Friday to the U.K. in 2010, says it will offer 50% more discounts than last year.

After several years of skepticism, traditional British retailers now view Black Friday as a vital part of their year. Upmarket department-store chain John Lewis had its busiest-ever day of online sales in 2013 and is expecting to beat it again this year. "It's become a great day in the calendar," said Mark Lewis, online director at John Lewis.

Still, some retailers this year are facing a familiar problem: many British shoppers still have no idea what Black Friday is or where it has come from.

A landing page on electronics retailer Currys's website is a black background with the words "Black Friday 2014, coming soon..." written in a stark white font. A countdown clock ticks off the seconds left until the big day.

Underneath the clock, a helpful message in smaller writing reads: "The Black Friday shopping deal phenomenon began in the U.S. It's always the Friday after Thanksgiving Day and is seen as the kickoff for the shopping season leading up to Christmas."

Write to Peter Evans at peter.evans@wsj.com

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